Compounds and methods of fabricating compounds exhibiting giant magnetoresistence and spin-polarized tunneling

ABSTRACT

The invention relates to inorganic intermetallic compounds having a PMR effect (combined GMR/CMR effect), which are characterized in that they contain at least two elements per formula unit and have a field sensitivity of less than 10% per 0.1 T at temperatures greater than 290 K. The invention also relates to composites consisting of these compounds, to a method for the production thereof and to their use, in particular, as magnetic field sensors or in the domain of spin electronics.

BACKGROUND

1. Technical Field

The present invention relates to compounds suitable for use inmagnetoelectronics.

2. Description of Related Art

Magnetoelectronics is a new field of electronics involving electroniccomponents that employ magnetoresistance effects and spin-polarizedelectrons, or will employ them in the future. Due to theirmagnetoresistive properties, such compounds may be employed as, amongother things, magnetic-field sensors. The “magnetoresistive properties”involved are a change in electrical resistivity induced by an externalmagnetic field (magnetoresistance). Due to the spin-polarized tunnelingthat occurs at room temperature, such compounds may also be employed asbasic building blocks in fabricating magnetic random-access memories(MRAM) and spin transistors. “Spin-polarized tunneling” is defined astunneling (quantum-mechanical penetration of a potential barrier) ofelectrons, where the probability that tunneling will occur depends upontheir spin polarization.

Magnetic-field sensors are used on the magnetic heads of hard-diskdrives that are employed as, e.g., external computer storage devices.Conventional magnetic heads detect the strengths and directions ofmagnetic fields based on a current induced in a coil. However, asrecording density increases, the space available for recording a bitdecreases, and the resultant magnetic fields will gradually decrease.Ultraresponsive magnetic-field sensors that cannot be manufactured usingconventional technologies are required for detecting such weak externalmagnetic fields. Ultraresponsive magnetic-field sensors that utilizegiant magnetoresistance (GMR) or tunneling magnetoresistance (TMR) areknown (cf. S. Mengel: “Innovationspotential Magnetoelektronik,”Physikalische Blätter 55 (3) (1999), pp. 53-56). Magnetic-field sensorsthat exhibit magnetoresistive effects at temperatures around 200° C. areemployed in the automotive industry. Although utilizing colossalmagnetoresistance (CMR) in compounds, such as manganese oxides, allowsattaining effects that are several orders of magnitude greater, thatapproach is not yet of technological interest due to the low fieldresponsivities that result.

SUMMARY

One aspect of the invention is an inorganic intermetallic compoundexhibiting a PMR-effect that contains at least two chemical elements performula unit and has an intrinsic field responsivity >10% per 0.1 T attemperatures in excess of 290 K.

Compounds in accordance with examples of the invention, in the form ofeither bulk materials or composites, may exhibit the followingbeneficial characteristics:

large negative magnetoresistances (MR₀>10%, preferably >20%, and,particularly preferred, >80% (80%, referred to their resistance in theabsence of a magnetic field, or 700%, referred to their saturatedmagnetization)), which are thus much larger than those for GMR-systems,at temperatures exceeding room temperature (preferably at the typicaloperating-temperature range of read-heads, i.e., around 25° C. to around55° C.),large magnetoresistive effects over a broad temperature range having abreadth of >100° C., preferably >200° C., and, particularlypreferable, >400° C., high field responsivities (>10% per 0.1 T,preferably >20% per 0.1 T, and, particularly preferable, >70% per 0.1T),a PMR-effect (>5% per 0.1 T, preferably >20% per 0.1 T, and,particularly preferable, >50% per 0.1 T) at room temperature,high resistances to thermal decomposition (up to temperatures of 50° C.,preferably up to 80° C., and, particularly preferable, up to 100° C.)and high chemical stabilities (resistance to H₂O, O₂, and, particularlypreferable, resistance to acids and alkalis),compatibility with silicon-processing technologies, andhigh spin polarizations (>60%, preferably >70%, and, particularlypreferable, >90%) at the Fermi energy in order to provide for theirbroad applicability in magneto electronics.

The invention also provides a number of other advantages and benefits,which should be apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows graphs of resistance measurements over a range oftemperatures, in the absence of an external magnetic field, and in thepresence of an external magnetic field, in accordance with an exemplaryembodiment of the invention.

FIG. 2 is a graph of magnetoresistance measurements over a range ofmagnetic field strengths, in accordance with an exemplary embodiment ofthe invention.

FIG. 3 is a graph of resistance measurements over a range of magneticfield strengths, in accordance with an exemplary embodiment of theinvention.

DETAILED DESCRIPTION

Devices based on GMR and TMR form the basic building blocks for greatlyimproved MRAMs. MRAMs combine the benefits of semiconductor memories(rapid access) and magnetic materials (high storage densities).Furthermore, the reading process is nondestructive, and magneticmemories are thus nonvolatile. Magnetic memories are rugged, require noexternal power supplies, and withstand radiation.

In principle, magnetoelectronic effects allow fabricating magneticallyswitchable spin transistors for electronic circuits have a new type ofdesign, where TMR-devices and/or hybrid magnetic-semiconductor devicesmight be employed for configuring transistor circuitry. Spinfield-effect transistors (Spin-FET) are more simply constructed than theusual metal-oxide semiconductor field-effect transistors (MOSFET), andboth of their states (transconducting and nontransconducting) arenonvolatile. Multivalued-logic devices might be configurable bycombining magnetic metallic chemical elements with semiconductingchemical elements (cf. G. Bayreuther and S. Mengel: “Magnetoelektronik,Grundlagenforschung—Zukunfts—technologie?,” VDI-Technologiezentrum(1998)). A metal or semimetal having a high spin polarization might be ahighly beneficial choice for fabricating either TMR-devices or spintransistors. Hybrid magnetic-material-semiconductor structures are basedon the idea that a polarizer-analyzer combination for the spins ofcharge carriers similar to those employed in optics might be configuredusing magnetic films. The resistance of the resultant hybrid devicewould vary with the relative orientations of the magnetizations of itstwo electrodes. However, in order to achieve that, it will be necessary,firstly, to supply spin-polarized currents by injecting spin-polarizedcharge carriers via the semiconductor layer and, secondly, to providethat the spin polarization of those currents is retained over a distancelong enough to allow them to reach the analyzer. Although the latterrequirement has already been met, there have been no reports ofunoxidized metallic materials that have high (in excess of 70%) spinpolarizations at room temperature.

The importance of magnetoresistive materials to magnetic sensors andmagnetic data storage has grown immensely over recent years, and suchmaterials have already become of major commercial significance.Materials suitable for fabricating products exhibiting GMR-effectsincorporate virtually exclusively materials involving 3d-metals, i.e.,metals having incomplete 3d-orbitals, since such materials exhibit thedesired magnetic properties and are largely compatible with thesilicon-processing techniques. Their major characteristics, such as themagnitudes of their magnetoresistive effects, their maximum operatingtemperatures (the operating temperatures of the resultant magnetic-fieldsensors), field responsivities, etc., have been improved over the pastfew years, although no optimal solution covering all application areashas yet been found.

The aforementioned GMR-effect occurs in multilayer systems. In thesimplest case, such multilayer systems consist of a pair of magneticfilms, e.g., a pair of iron films, separated by a nonmagnetic film,e.g., a chromium film. The GMR-effect, which has been known since 1987,is presumably an extrinsic interface effect (in the case of multiphasesystems) (cf. German Patent DE-A 38 20 475). The resistance of suchsystems will be low whenever the pair of iron films areferromagnetically coupled by the chromium film, since electrons willthen be able to transit from one iron film to the other without alteringtheir spins. On the other hand, the resistance of such systems will behigh if the spins of the two iron films are antiferromagneticallycoupled. “Ferromagnetism” is generally defined as a collectivemagnetism, under which electron spins are aligned parallel to oneanother for temperatures below the Curie temperature.“Antiferromagnetism” is defined as a collective magnetism, under whichelectron spins are aligned antiparallel to one another for temperaturesbelow the critical temperature (the Neel temperature). The resistance ofan antiparallel-coupled multilayer system, measured between its ironfilms, may be significantly reduced by an externally applied magneticfield. The external magnetic field forces a ferromagnetic alignment ofthe spins of both iron films along the field direction. This effect maybe utilized to achieve an MR₀-effect of at most 10% at room temperature.The GMR-effect is currently already at the engineering-applicationstage, in particular, for fabricating magnetic-field sensors for theread-heads of hard-disk drives (at IBM, Mainz).

Magnetoresistance is a parameter that describes the percentage change inthe resistance of a system in the presence of, and in the absence of, amagnetic field. A “negative magnetoresistance” is defined as a reductionin electrical resistance that occurs in the presence of an externalmagnetic field, relative to that when no magnetic field is present. Twodiffering definitions of “percentage magnetoresistance” are in generaluse. The definition of “percentage magnetoresistance,” MR₀, that will beemployed in the case of the present invention is the difference betweenthe resistance of a device in the presence of a magnetic field and itsresistance in the absence of a magnetic field, divided by its resistancein the absence of a magnetic field. Its maximum value is 100%. Analternative definition (e.g., that employed in German Patent DE-A 38 20475) is employed in the case of the giant-magnetoresistance (GMR)effect. In this case, “percentage magnetoresistance,” MR_(P), is definedas the difference between the resistance of a device in the absence of amagnetic field and its resistance in the presence of a magnetic field,divided by its resistance when the magnetic field is present. Itsmaximum value may thus be arbitrarily large.

The aforementioned TMR-effect is based on an insulating film sandwichedbetween a pair of magnetic films. If this insulating film issufficiently thin, electrons will be able to tunnel through theresultant potential barrier and current will be able to flow between thepair of magnetic films.

The tunneling probability is also spin-dependent, which leads to highmagnetoresistances for parallel magnetization of the spins in themagnetic tunneling-contact films, compared to those for antiparallelmagnetization of their spins. Materials that have exclusivelyspin-polarized electrons would exhibit a large effect. Since thetunneling probability is spin-dependent, in view of the relation

${{M\; R} = \frac{P}{1 - P^{2}}},$

the magnetoresistance, MR, will be a maximum when all spins, P, have thesame polarization. However, the aforementioned magnetoresistances aremeasurable in chemical elements and thin films of chemical elementsonly.

Large magnetoresistances were discovered in compounds as well in 1993.Discovery of the “colossal magnetoresistance” (CMR) effect in manganeseoxides (cf. R. von Helmold, J. Wecker, B. Holzapfel, L. Schulz, and K.Samwer, Phys. Rev. Lett. 71 (1993), p. 2331) attracted worldwideinterest, since the change in their resistance when an external magneticfield was applied was much greater than for the aforementionedmultilayer systems composed of chemical elements. The CMR-effect is anintrinsic effect (an effect occurring within a chemical compound), andthe extremely large change in resistance is due to suppression of ametal-insulator transition at the Curie temperature, T_(C). The “Curietemperature” is defined as the critical temperature, below which aspontaneous magnetization involving a parallel alignment of spin momentson those of neighboring atoms, which is also termed a “ferromagneticalignment,” occurs. At temperatures above T_(C), spins will be randomlyoriented, and compounds that exhibit a CMR-effect will be insulators(semiconductors). At temperatures below T_(C), they will beferromagnetic metals. The CMR-effect was first measured formixed-valency manganese oxides, such as La_(1-x)Sr_(x)MnO₃. The negativeMR that occurs in the case of CMR is attributable to the reduction inspin disorientation that occurs. The electrical conductivity that occursis due to e_(g)-electrons “hopping” between Mn³⁺-sites and Mn⁴⁺-sites,and that “hopping” occurs only when the magnetic moments of the twoMn-atoms are aligned parallel to one another, as in the ferromagneticcase. A metal-insulator transition occurs at the Curie temperature. Inthe presence of an applied magnetic field, the probability of “hopping”will increase as the degree of ferromagnetic alignment increases, andresistance will decrease. The effect is thus usually greatest at theCurie temperature. Although engineering applications of the CMR-effectare also thought to be feasible, the greatest effect is usually observedat temperatures below room temperature, rather than at temperaturesfalling within the operating-temperature ranges of read-heads. Fieldresponsivities are also still too low, and epitaxial growth of thecompounds involved on silicon wafers continues to present problems.

Polycrystalline samples or highly compacted pellets of compoundsexhibiting the CMR-effect and CrO₂ exhibit a so-called“powder-magnetoresistance” (PMR) effect (cf. A. Gupta and J. Z. Sun:“Spin-polarized transport and magnetoresistance in magnetic oxides,” J.Magn. Magn. Mat. 200 (1999), pp. 24-43). Far below their Curietemperature, these compounds exhibit high polarizations of electrons atthe Fermi energy, which leads to samples that have grain boundariesexhibiting large magnetoresistances in weak magnetic fields (as much as20% at 4 K and 0.1 T). Only some of their totally spin-polarizedelectrons are able to tunnel to states of neighboring grains that havedifferently aligned spins. An insulating oxide film probably forms thetunneling barrier. Their measured resistance is thus large. An externalmagnetic field will align all spins and reduce their resistance, sincetheir conduction electrons will then be able to tunnel to neighboringgrains. Nevertheless, all compounds that have become known to dateexhibit no noticeable effect at room temperature. Since all suchcompounds are also oxides, transferring these devices to wafers presentsproblems. The largest effect at room temperature (6% at 1 T; cf. K.-I.Kobayashi, Nature 395 (1998), p. 677) was measured in Sr₂FeMoO₆.Composites composed of a metallic material having a high spinpolarization and an insulating, or at least semiconducting, material,e.g., an oxide, such as Al₂O₃, MgO, or Cr₂O₃, a fluoride, such as MgF₂,or some other insulating, or semiconducting, compound, such as a polymeror oligomer, such as polystyrene, polyethylene, an epoxy resin, etc,should be suitable materials for use in magnetoelectronics due to theirPMR-effects. The insulating material prevents short-circuiting of themetallic particles, and probably also serves as a tunneling barrier.

The problem addressed by the present invention is making availablematerials that will allow realizing higher (up to 8 Gbits/cm²) recordingdensities on hard-disk drives than in the case of the hard-disk drivesin current use, which are based on a GMR-effect or the TMR-effect andallow reaching recording densities of about 1 Gbit/cm². The materialsinvolved should also exhibit high spin polarizations at room temperaturein order to allow their employment as TMR-devices and in spinelectronics. High magnetoresistances at temperatures in excess of roomtemperature are necessary if they are to be employed as magnetic-fieldsensors in the automotive industry.

There thus existed a need for materials that will exhibit the followingproperties when combined:

-   -   the favorable characteristics of multilayer systems that exhibit        a GMR-effect, namely:        -   high field responsivities,        -   a large effect at the operating temperatures of read-heads,        -   a stable effect over a broad temperature range, and    -   the favorable characteristics of compounds that exhibit a        CMR-effect, namely:        -   an extremely large effect in a bulk material or a composite            composed of an insulating material and a metallic material            exhibiting a high spin polarization, where the effect will            not be strongly dependent upon the thickness of the            intervening film, as in the case of the GMR-effect, and        -   for use in spin electronics, high spin polarization at            temperatures as close as possible to room temperature.

The present invention now makes available compounds that combine thefavorable characteristics of the GMR-effect and CMR-effect with oneanother and exhibit high spin polarization at room temperature. Theirmagnetoresistances exceed those of GMR-systems and TMR-systems by afactor of about three, while providing field responsivities better thanthose of manganese oxides. Although sensors based on a GMR-effect orTMR-effect employed on hard-disk drives allow reaching recordingdensities of about 1 Gbit/cm², compounds according to the inventionshould allow reaching higher recording densities (up to 10 Gbits/cm²).In composite form (granular materials composed of the compound accordingto the invention and an insulating, or at least semiconducting,material) magnetoresistive effects of as much as 80% (80% of theirresistance in the absence of a magnetic field, or 700% of theirsaturation magnetization) have been measured.

The invention has been based on the following theoreticalconsiderations: An intermetallic compound incorporating 3d-metals asconstituents might meet the above requirements, since such compoundsmight simultaneously exhibit the desired properties of both themetallic, multilayer systems and compounds that exhibit ametal-insulator transition. It will be beneficial if their Curietemperature will continue to exceed room temperature (23° C.) in orderthat the largest effect will be attained for temperatures falling withinthe operating-temperature range of read-heads. A Curie temperature inexcess of 300° C. would be favorable for spin-polarized tunneling andfor the PMR-effect at room temperature. The compounds according to theinvention exhibit the effect, virtually independent of temperature, overthe temperature range investigated, i.e., temperatures up to 400 K.

An understanding of the theoretical model that describes the physicalbasis for the CMR-effect will be useful in synthesizing compoundsmeeting the aforementioned requirements profile. Whether a compound isan insulator, i.e., a material having an even number of valenceelectrons, or a metal, i.e., a material having an odd number of valenceelectrons, is a matter of conjecture. If certain 3d-metals or 4f-metalsare constituents of a compound, then statements regarding its magnetismmay be inferred from the distance between those atoms in the solidstate. That such estimates are possible is due to the existence of arelation among the crystalline structure of such a compound, itsvalence-electron concentration, and its electronic and magneticproperties.

“Valence-electron concentration” is defined as the number of valenceelectrons per atom in the formula unit involved. “Valence electrons” aredefined as electrons in orbitals of chemical elements outside the last,closed, inert-gas shell. In the case of the transition metals, theirvalence electrons are s-electrons, p-electrons, and d-electrons, and, inthe case of the lanthanides, f-electrons as well.

The electronic structure of a solid may be described in terms of itsband structure, and may currently be computed with sufficient accuracy.“Band structure” is the solid-state analog of the energy levels ofmolecules. The electronic structures of solids may be measured usingspectroscopic methods, such as photoemission spectroscopy.

Systematic study of the electronic structures of known compounds havingthe aforementioned electronic and magnetic properties has allowed, forthe first time, formulating an initial recipe for synthesizing compoundsexhibiting giant magnetoresistances using a “fingerprint” (cf. C.Felser, R. Seshadri, A. Leist, and W. Tremel, J. Mater. Chem. 8 (1998),p. 787, and C. Felser and R. Seshadri, J. Inorg. Mater. 2 (RaveauCommemorative Volume, 2000), p. 677).

The GMR-effect in the aforementioned Fe—Cr—Fe-system may also beexplained on the basis of the electronic structure of chromium and thespin-density wave (to be explained below) and magnetism of iron (cf. P.Bruno and C. Chappert, Phys. Rev. B 46 (1992), p. 261).

The “double-exchange” (“double Zener-exchange”) mechanism (cf. C. Zener,Phys. Rev. 82 (1951), p. 403) is usually invoked in order to explain thegiant magnetoresistance of the manganese oxides, the CMR-effect.However, that approach to an explanation is confined to themixed-valency manganese oxides. There is no general theory.

Spin-density waves, which are largely responsible for the GMR-effect,are also under discussion as the probable cause of high-temperaturesuperconductivity (cf. G. Burdett: Chemical Bonding in Solids. OxfordUniversity Press (1995)). However, superconducting cuprates andmanganese oxides exhibiting the CMR-effect are structurallyinterrelated. Spin-density waves were thought to be coresponsible forgiant magnetoresistances as long ago as 1998 (cf. C. Felser, R.Seshadri, A. Leist, and W. Tremel, J. Mater. Chem. 8 (1998), p. 787).Spin-density waves are consequences of the unusual electronic structuresof manganese oxides. Computations of the electronic structures of thesecompounds based on band-structure computations allow recognizing whethera given compound will exhibit a spin-density wave. Theoretically, oneneeds a four-dimensional plot involving the three spatial coordinates(in momentum space) and the energy in order to represent athree-dimensional plot of the electronic structure of a solid. One meansfor arriving at a three-dimensional representation is using theisoenergetic surface, a surface of constant energy (in momentum space).The isoenergetic surface, which separates occupied states fromunoccupied states, is the Fermi surface, and the associated energy isthe Fermi energy. Under this representation of electronic structure,spin-density waves are identifiable as parallel surfaces on theisoenergetic surface. Due to the symmetry of their crystallinestructures and their valence-electron concentrations, manganese oxidesexhibit such a “fingerprint” in their electronic structures. Magneticmanganese oxides that exhibit a CMR-effect also exhibit an additional,local, magnetic moment. “Compounds exhibiting a local magnetic moment”are defined as compounds having unpaired electrons, paramagneticcompounds whose spins are magnetically aligned at temperatures below thecritical temperature, T_(C). Local magnetic moments in manganates arecaused by a partially filled electron shell, in this case, thehalf-filled t_(2g)-shell of manganese.

Formation of an antiferromagnetic arrangement may occur in the case ofcompounds as well, depending upon the ratio of the wavelength of thespin-density wave (a result of their band structure) to the distancebetween magnetic atoms in the solid. This “fingerprint,” i.e.,spin-density waves, has been observed in superconductors and magneticsystems exhibiting negative magnetoresistances. Knowledge gained fromthe electronic structures of superconducting compounds and consistentapplication of the model have led to prediction, and subsequentexperimental verification, of a CMR-effect in GdI₂ (cf. C. Felser and R.Seshadri, J. Mater. Chem. 9 (1999), p. 459, C. Felser, K. Thieme, and R.Seshadri, J. Mater. Chem. 9 (1999), p. 459, and C. Felser, K. Ahn, R. K.Kremer, and R. Seshadri, and A. Simon, J. Solid State Chem. 147 (1999),p. 19). GdI₂ becomes ferromagnetically arranged at 290 K and exhibits aCMR-effect having a magnetoresistance, MR₀, of 70% at room temperatureand 7 T. Although this compound was the world-record holder at roomtemperature until the compound according to the invention wassynthesized, engineering applications were precluded due to its chemicalreactivity (it reacts with H₂, H₂O, and O₂) and its low fieldresponsivity.

A new, greatly improved, model has meanwhile been developed. In additionto the aforementioned spin-density wave, a saddle point in the bandstructures of compounds exhibiting the CMR-effect is beneficial. Asaddle point, conforming to the mathematical definition of the term, intheir spin-polarized band structure leads to a high state density.“State density” is defined as the total number of energy levels fallingwithin a specified energy interval. A high state density (more thanthree energy levels per elementary cell and eV) at the Fermi energy, theenergy that separates occupied states from unoccupied states, isunfavorable for the electronic system and is thus termed an “electronicinstability.” Incidence of these two types of instability, spin-densitywaves and saddle points, at the Fermi energy is favorable for theground-state properties of compounds having unusual electronic andmagnetic properties. The “fingerprint” for compounds exhibiting giantmagnetoresistance thus consists of three factors: a saddle point, aspin-density wave, and a local magnetic moment. The saddle pointapparently leads to a ferromagnetic coupling among magnetic atoms (cf.Hedin, J. Magn. Mat. 177 (1998), p. 599), while the spin-density waveleads to an antiferromagnetic arrangement of the spins of neighboringatoms. This competition between ferromagnetic and antiferromagneticcoupling appears to be the cause of the large change in resistance atthe Curie temperature, and is thus responsible for the giantmagnetoresistance that occurs in compounds.

If the materials involved also exhibit high spin polarizations at theFermi energy, magnetoresistance effects will be particularly large.“Spin polarization at a specified energy” is defined as the ratio of thestate densities for the two spin orientations at that energy. Themaximum value of spin polarization is unity (a nonmagnetic compound hasa spin polarization of zero), and magnetoresistance, which, in the caseof the PMR-effect and the TMR-effect, would be dependent upon spinpolarization, will be maximized, in view of the relation

${M\; R} = {\frac{P}{1 - P^{2}}.}$

Furthermore, in addition to the aforementioned, trifactor (saddle point,spin-density wave, and local magnetic moment) “fingerprint,” furtherrequirements have, appropriately, arisen. Although, instabilitiesfrequently occur in the band structures of solids, in fact, they willaffect their electronic properties only if they occur in the vicinity ofthe Fermi energy. Although a shift in the Fermi energy can,theoretically, be achieved by doping a compound with electrons or holes,in practice, it frequently is difficult to achieve. The Fermi energy maybe shifted toward higher energies by doping with electrons, and shiftedtoward lower energies by doping with holes, which will allowrepositioning the instability in band structure precisely on the Fermienergy. Doping a compound, such as SrMnO₃, with electrons involvesreplacing the corresponding fraction of strontium atoms by lanthanumatoms. The only candidates for such replacements are thus compoundsystems that may be doped, i.e., that exist for various combinations ofchemical elements and in which individual atoms may be replaced by otheratoms.

Intermetallic compounds, such as Heusler compounds, accurately meet theprerequisites demanded:

a trifactor “fingerprint” in their band structure, consisting ofa saddle point,a spin-density wave, anda local magnetic moment.

They are semimetallic, ferromagnets having

a variable valence-electron concentration,highly symmetric structures, andCurie temperatures >500 K.

The prerequisites for potential occurrence of an MR-effect are:

a high field responsivity,a large effect at the operating temperatures of read heads,stability of the effect over a broad temperature range, andan extremely large effect in a bulk or composite material, where theeffect should not be strongly dependent upon the thickness of theintervening film, as in the case of the GMR-effect.

In the case of applications to spin electronics: high spin polarizationat temperatures close to room temperature.

Intermetallic compounds are a reasonable choice for attaining high fieldresponsivities in the case of compounds exhibiting giantmagnetoresistances and, in particular, semimetallic ferromagnets will befound among the Heusler phases. “Heusler phases” are intermetalliccompounds having the general composition X₂YZ, and crystallize into theBiF₂ type of structure (cf. Pearson's Handbook of Crystallographic Datafor Intermetallic Phases. ASM International, The Materials InformationSociety (1991)). Included among intermetallic compounds are compoundsinvolving

a) two or more true metals (T₁ and T₂),b) one or more true metals and one or more true metals of theB-subgroups, andc) two or more metals of the B-subgroups,where their properties become less metallic and more like those of truechemical compounds when they undergo a transition from Class 1 to Class3. The classification into true metals and chemical elements of theB-subgroups is based on Table 13.1 of R. C. Evans: Einführung in dieKristallchemie. Walter de Gruyter Verlag, Berlin and New York (1976), p.276:

The lanthanides and actinides belong to Class T₂.

Cubic structures of the BiF₂-type are characterized by four internestingfcc-lattices. X and Y are usually transition metals (T₂ in the tableabove). Y is usually a rare-earth element. Z is a nonmagnetic metal ornonmetal (an element from Subgroup B₁ or B₂). Due to their magneticbehavior, the magnetooptic Kerr effect, Heusler phases have acquiredtechnological importance. The magnetic properties of most known Heuslerphases have thus been rather well investigated. In particular, theirferromagnetic Curie temperatures and magnetic moments are known.However, their electronic properties, such as their electricalconductivities, have been investigated in rare cases only. With theexception of the recently discovered semiconducting, nonmagnetic,Fe₂Val, all Heusler compounds that have thus far become known, i.e.,several hundred distinct phases, are metals. Several ferromagneticHeusler compounds have band structures characteristic of semimetallicferromagnets. The high degrees of symmetry of their cubic structureshave also proven favorable, due the occurrence of electronicinstabilities. Heusler phases exist over a relatively broad range ofvalence-electron concentrations, which means that doping them withelectrons or holes, as required, should present no problems. Since themagnetic compounds have been rather thoroughly investigated, the Curietemperatures of unknown, new, or doped compounds may be inferred fromthe ferromagnetic Curie temperatures of known compounds. Band-structurecomputations for various Heusler compounds have shown that this class ofcompounds either immediately meets the aforementioned criteria, or willmeet them if their valence-electron concentrations are suitably adjustedby varying their elemental compositions. For a valence-electronconcentration of 6.95±0.5, preferably 6.95±0.2, in particular, 6.95±0.1,and, much to be preferred, 6.95±0.02, valence electrons per atom intheir formula unit, the instabilities involved (saddle points andspin-density waves) occurred at their Fermi energy. Other instabilitiesin Heusler compounds occur for valence-electron concentrations of5.5±0.5, 6.95±0.5, preferably 5.5±0.2, in particular, 5.5±0.1, and, muchto be preferred, 5.5±0.02, valence electrons per atom in their formulaunit, and 7.13±0.5, preferably 7.13±0.2, in particular, 7.13±0.1, and,much to be preferred, 7.13±0.02, valence electrons per atom in theirformula unit.

Since known Heusler compounds do not exhibit such valence-electronconcentrations (VEC) under all conditions, in particular, under allauxiliary conditions (e.g., a Curie temperature falling within the rangeapplicable to read-heads), doping of the known compounds was necessary,where it was assumed that their band structure would not be altered bydoping and only their Fermi energy would be shifted (the“rigid-band-model” assumption). In fact, Heusler compounds synthesizedbased on the aforementioned theory, Co₂Cr_(0.6)Fe_(0.4)Al (VEC=6.95),Co₂Cr_(0.6)Fe_(0.4)Ga (VEC=6.95), and Co₂Cr_(0.2)Mn_(0.8)Al (VEC=6.95),subsequently exhibited the expected giant magnetoresistances. Theinvestigations of their electronic properties that were conducted showedthat, as had been postulated, compounds according to the invention,Co₂Cr_(0.6)Fe_(0.4)Al, Co₂Cr_(0.6)Fe_(0.4)Ga, and Co₂Cr_(0.2)Mn_(0.8)Al,combined the favorable properties of the GMR-effect (high fieldresponsivities) and the CMR-effect (a large effect in a compound).Heusler phases according to the invention exhibit magnetoresistances ofthe same order of magnitude as those of the manganites (the“colossal-magnetoresistance effect” (CMR-effect)) and fieldresponsivities as high as those of the multilayer systems (the“giant-magnetoresistance effect” (GMR-effect)). Since measurements ofthe effect were conducted on polycrystalline samples, it may beconcluded that spin polarization was nearly 100%. Other compoundsaccording to the invention are, for example, Co₂Mn_(0.8)Cr_(0.2)Al, andCo₂Mn_(0.8)Cr_(0.2)Ga, for both of which VEC=6.95, andCo₂MnGe_(0.5)Ga_(0.5), Co₂MnSi_(0.5)Al_(0.5), Co₂MnSn_(0.5)Sn_(0.5),Co₂Mn_(0.5)Cr_(0.5)Si, Co₂Mn_(0.5)Fe_(0.5)Al, Co₂Mn_(0.5)Fe_(0.5)Ga,Co₂Mn_(0.5)Fe_(0.5)In, Fe₂CoGe_(0.5)Ga_(0.5), and Fe_(2.5)Co_(0.5)Ga,for all of which VEC=7.13.

Compounds according to the invention are thus inorganic, intermetalliccompounds that contain at least two chemical elements per formula unitand exhibit negative magnetoresistances having field responsivities >5%per 0.1 T, in particular, >10% per 0.1 T, preferably >20% per 0.1 T,and, much to be preferred, >70% per 0.1 T, at room temperature. Admixingoxides enhances the PMR-effect. Preferred compounds according to theinvention are intermetallic compounds that exhibit cubic symmetry,preferably having few, or no, structural imperfections. “Structuralimperfections” are defined as lattice parameters that differ by <10%, inparticular, by <5%, and preferably by <2%. In addition, it has proven tobe beneficial if the compounds belong to the Heusler phases. Compoundsaccording to the invention exhibit electronic instabilities (saddlepoints, spin-density waves, and local magnetic moments) in the vicinityof the Fermi energy. An “instability in the vicinity of the Fermienergy” is defined as an instability differing from the Fermi energy byno more than ±0.5 eV, preferably no more than ±0.2 eV, in particular, nomore than ±0.1 eV, and, much to be preferred, no more than ±0.02 eV. Itwill be beneficial if compounds according to the invention exhibitingthe combined CMR/GMR-effect exhibit that effect at temperatures inexcess of room temperature (23° C.). Such compounds are characterized byvalence-electron concentrations of 6.95±0.5, preferably 6.95±0.2, inparticular, 6.95±0.1, and, much to be preferred, 6.95±0.02, electronsper atom in their formula unit. Other instabilities in Heuslercompounds, and thus other preferred VECs, are observed forvalence-electron concentrations of 5.5±0.5, preferably 5.5±0.2, inparticular, 5.5±0.1, and, much to be preferred, 5.5±0.02, electrons peratom in the formula unit, and 7.13±0.5, preferably 7.13±0.2, inparticular, 7.13±0.1, and, much to be preferred, 7.13±0.02, electronsper atom in their formula unit. The optimized valence-electronconcentrations of specific compounds may be determined from accurateband-structure computations. Optimal properties have thus far beencomputed for the compounds Co₂Cr_(0.6)Fe_(0.4)Al, Co₂Cr_(0.6)Fe_(0.4)Ga,and Co₂Cr_(0.2)Mn_(0.8)Al, and have also been observed. As a samplecomputation, Co has 9, elemental Cr has 6, Fe has 8, and Al has 3valence electrons. A stoichiometric composition of Co₂Cr_(0.6)Fe_(0.4)Alyields (2×9)+(0.6×6)+(0.4×8)+3=27.8 valence electrons per formula unitand, following division by 4, 6.95 valence electrons per atom. Thesevalence electrons exhibit a local magnetic moment for a Curietemperature exceeding room temperature (23° C.). The preferred compoundsaccording to the invention, Co₂Cr_(0.6)Fe_(0.4)Al,Co₂Cr_(0.6)Fe_(0.4)Ga, and Co₂Cr_(0.2)Mn_(0.8)Al, are semimetallicferromagnets, which is why their measured effects are particularlylarge. Composites according to the invention, which exhibit aPMR-effect >10% per 0.1 T, in particular, >20% per 0.1 T,preferably >50% per 0.1 T, and, much to be preferred, >80% per 0.1 T, atroom temperature are granular materials consisting of compoundsaccording to the invention and an insulating material, such as Al₂O₃, inadmixtures containing mole fractions of, for example, 15%±15%,preferably 15%±10%, in particular, 15%±5%, and, much to be preferred,15%±2%, of the latter. Magnetoresistive effects ranging up to 80%,referred to their resistance in the absence of a magnetic field, or upto 700%, referred to their saturated magnetization, have been measuredfor such materials. The effects remain stable over broad temperatureranges whose breadths exceed 50° C., in particular, exceed 100° C.,preferably exceed 200° C., and, much to be preferred, exceed 400° C.,i.e., the effects vary by <50%, in particular, <20%, and, much to bepreferred, <10%, over those temperature ranges. These materials, eitherin the form of bulk materials embedded in other materials, such as epoxyresins or polymers, or in the form of granular films, may be employed“as is” as magnetic-field sensors for use in magneto electronics.

Compounds according to the invention, in the form of either bulkmaterials or composites, exhibit

large negative magnetoresistances (MR₀>10%, preferably >20%, and,particularly preferred, >80% (80%, referred to their resistance in theabsence of a magnetic field, or 700%, referred to their saturatedmagnetization)), which are thus much larger than those for GMR-systems,at temperatures exceeding room temperature (preferably at the typicaloperating-temperature range of read-heads, i.e., around 25° C. to around55° C.),large magnetoresistive effects over a broad temperature range having abreadth of >100° C., preferably >200° C., and, particularlypreferable, >400° C.,high field responsivities (>10% per 0.1 T, preferably >20% per 0.1 T,and, particularly preferable, >70% per 0.1 T),a PMR-effect (>5% per 0.1 T, preferably >20% per 0.1 T, and,particularly preferable, >50% per 0.1 T) at room temperature,high resistances to thermal decomposition (up to temperatures of 50° C.,preferably up to 80° C., and, particularly preferable, up to 100° C.)and high chemical stabilities (resistance to H₂O, O₂, and, particularlypreferable, resistance to acids and alkalis),compatibility with silicon-processing technologies, andhigh spin polarizations (>60%, preferably >70%, and, particularlypreferable, >90%) at the Fermi energy in order to provide for theirbroad applicability in magneto electronics.

Compounds according to the invention may be assembled from two or morechemical elements, where the types and quantities of the chemicalelements involved are chose such that the resultant compoundscrystallize into cubic crystals whose lattice constants differ by ≦10%,preferably by ≦5%, in particular, by ≦2%. To a first approximation,their cubic symmetries are determined by the ratios of the atomic radiiof the atoms involved (cf. R. C. Evans: Einführung in dieKristallchemie. Walter de Gruyter Verlag, Berlin and New York (1976), p.276). Ideally, the chemical elements and stoichiometry to be involvedare chosen such that the resultant compounds belong to the Heuslerphases. The chemical elements chosen should exhibit an electronicinstability (saddle point, spin-density wave) in the vicinity, i.e.,within ±0.5 eV, preferably within ±0.2 eV, and, particularly preferable,within ±0.1 eV, of the Fermi energy. If that should not be the case, thecompounds should beneficially be doped with electrons or holes, inaccordance with their theoretically computed electronic structures. Thecombinations of chemical elements chosen should preferably yieldcompounds that have a valence-electron concentration of 6.95±0.5,preferably 6.95±0.2, in particular, 6.95±0.1, and, much to be preferred,6.95±0.02. Alternative valence-electron concentrations are 5.5±0.5,preferably 5.5±0.2, in particular, 5.5±0.1, and, much to be preferred,5.5±0.02, and 7.13±0.5, preferably 7.13±0.2, in particular, 7.13±0.1,and, much to be preferred, 7.13±0.02, valence electrons per atom in theformula unit. It would be beneficial if the resultant compounds wouldhave a local magnetic moment and a Curie temperature exceeding roomtemperature. From these synthetic compounds, one then selects those thatare semimetallic ferromagnets, which will be those compounds that havestate densities for a single spin orientation only at their Fermienergy.

In the following, the invention will be explained in greater detail,based on examples.

EXAMPLES

The compounds involved are synthesized from the chemical elements to beinvolved, in accordance with their stoichiometric ratios. Measuredquantities are pressed into pellets and fused for around 30 seconds inan inert-gas atmosphere using an arc welder. It will be beneficial ifthis process is repeated several times in order to homogenize thesamples. The resultant weight losses will usually be less than 5%.Tempering the samples at high temperatures for short periods has beenfound to be beneficial. Samples in quartz ampoules that were tempered invacuum at 800° C. for 5 days exhibited reduced effects, although theirX-ray-diffraction patterns indicated no changes in their crystallinestructure. Their magnetoresistive properties were investigated byconducting electrical-resistance measurements, both in the presence ofan external magnetic field and in the absence of an external magneticfield. These measurements of electrical resistance on samples wereperformed in an Oxford Instruments helium cryostat using the four-pointmethod, and covered the temperature range 300 K to 4 K. R(T)-curves wererecorded for zero magnetic field and a magnetic field of 8 T. TheB-field dependence of their electrical resistance was also measured atselected temperatures for applied magnetic fields ranging from −8 T to+8 T. The fused, globular samples were ground using a mortar and pressedinto 8-mm-diameter pellets around 1 mm thick using an applied force of 5metric tons prior to the investigations of the behaviors of theirelectrical resistance.

Example 1 Co₂Cr_(0.6)Fe_(0.4)Al

A total of 2 g of this compound was synthesized from the chemicalelements involved (99.8-%-purity Co, supplied by Alfa Metals, Karlsruhe,99.8-%-purity Cr, also supplied by Alfa Metals, Karlsruhe,99.99-%-purity Al, supplied by Chempur, Karlsruhe, and 99.9-%-purity Fe,supplied by Alfa Metals, Karlsruhe) in accordance with itsstoichiometric ratios. Pellets were pressed from weighed quantities andfused for about 30 seconds in a 700-mbar argon atmosphere using an arcwelder operated at a current of 65 A and a voltage of 20 V. That processwas repeated three times in order to homogenize the samples. The weightlosses involved were less than 2%. The purity of the product was checkedusing Cu—K_(α)-radiation on a Siemens D5000 X-ray powder diffractometer.The Co₂Cr_(0.6)Fe_(0.4)Al-phase (a Heusler phase having aCo₂CrAl-structure and cubic symmetry, with a=0.5724 nm) exhibited noimpurities. Its R(T)-curve for 0 T is shown in FIG. 1. A plot of itselectrical resistance versus temperature in the absence of an externalmagnetic field exhibits a broad, local maximum at 300 K. Its electricresistance decreases as its temperature decreases until a temperature ofabout 150 K is reached and then increases as its temperature is furtherreduced, which is an indication of active behavior.

Applying an external magnetic field of 8 T reduces its resistance at alltemperatures and suppresses the sharp transition at 150 K, whichsmoothes out the local minimum and shifts it toward higher temperatures(cf. FIG. 1). Measurements of the magnetic-field dependence of itsmagnetoresistance at 4 K yielded a negative magnetoresistance of nearly10% at a magnetic flux density of 2 T that increased with increasingtemperature to 12% at 200 K and nearly 20% at 300 K (cf. FIG. 2).Measurements conducted over the range −0.1 T to +0.1 T yielded anextraordinarily high field responsivity for this compound. Saturationsat effects in excess of 20% and of 10% were obtained for magnetic fluxdensities of 0.1 T and about 0.03 T, respectively.

Example 2 Co₂Cr_(0.6)Fe_(0.4)Al+10%, by mole fraction, Al₂O₃

The compound Co₂Cr_(0.6)Fe_(0.4)Al was prepared as described underExample 1. The resultant product was finely ground using a mortar andhomogenized with 15%, by mole fraction, 99.9-%-purity Al₂O₃ supplied byChempur. This admixture was then pressed into a pellet.

The results of R(B)-measurements on this sample at 295 K are shown inFIG. 3. Observation of the field dependence of its magnetoresistance at295 K yielded a negative magnetoresistance of nearly 700%, referred toits saturation magnetization, at 0.1 T. Measurements conducted over therange −0.1 T to +0.1 T yielded an extraordinarily high fieldresponsivity for this compound. A saturation at an effect in excess of40% was measured for a magnetic flux density of 0.05 T.

Aspects of the invention are exemplified in the following entries:

-   Entry 1. An inorganic intermetallic compound exhibiting a PMR-effect    that contains at least two chemical elements per formula unit and    has an intrinsic field responsivity >10% per 0.1 Tat temperatures in    excess of 290 K.-   Entry 2. A compound according to entry 1 having cubic symmetry, and    whose lattice constants differ by no more than 10%.-   Entry 3. A compound according to entry 1 or entry 2 belonging to the    class of Heusler phases.-   Entry 4. A compound according to entry 1, entry 2, or entry 3 that    has a saddle point and a spin-density wave in its band structure in    the vicinity of the Fermi energy, both of which fall within ±0.5 eV    thereof.-   Entry 5. Compounds according to any of entries 1-4, for which the    CMR/GMR-effect occurs at room temperature (23° C.) and higher    temperatures.-   Entry 6. A compound according to any of entries 1-5 having a    valence-electron concentration of 6.95±0.5 per atom in its formula    unit.-   Entry 7. A compound according to any of entries 1-5 having a    valence-electron concentration of 5.5±0.5 per atom in its formula    unit.-   Entry 8. A compound according to any of entries 1-5 having a    valence-electron concentration of 7.13±0.5 per atom in its formula    unit.-   Entry 9. A compound according to any of entries 1-8 having a local    magnetic moment.-   Entry 10. A compound according to any of entries 1-9 whose Curie    temperature exceeds room temperature (23° C.).-   Entry 11. A compound according to any of entries 1-10 that is a    ferromagnetic semimetal.-   Entry 12. Compounds having the formulae Co₂Cr_(0.6)Fe_(0.4)Al,    Co₂Cr_(0.2)Mn_(0.8)Al, Co₂Cr_(0.6)Fe_(0.4)Ga, Co₂Mn_(0.8)Cr_(0.2)Al,    Co₂Mn_(0.8)Cr_(0.2)Ga, Co₂MnGe_(0.5)Ga_(0.5), Co₂MnSi_(9.5)Al_(0.5),    Co₂MnSn_(0.5)In_(0.5), Co₂Mn_(0.5)Cr_(0.5)Si, Co₂Mn_(0.5)Fe_(0.5)Al,    Co₂Mn_(0.5)Fe_(0.5)Ga, Co₂Mn_(0.5)Fe_(0.5)In, Fe₂CoGe_(0.5)Ga_(0.5),    or Fe_(2.5)Co_(0.5)Ga.-   Entry 13. A composite composed of at least one compound according to    entry 1 and at least one insulating or semiconducting substance.-   Entry 14. A composite according to entry 13, wherein the insulating    or semiconducting substance employed is employed in a mole fraction    of 15% or more, referred to the quantity of the compound according    to entry 1 employed.-   Entry 15. A composite according to entry 13 or entry 14, wherein the    insulating or semiconducting substance is chosen from one or more of    the following types of substances: oxides, fluorides, polymers, or    oligomers.-   Entry 16. A composite according to any of entries 13-15 that    exhibits a PMR-effect and has a field responsivity >10% per 0.1 T at    temperatures in excess of 290 K.-   Entry 17. A composite according to any of entries 13-16 whose    PMR-effect occurs at room temperature (23° C.) and higher    temperatures.-   Entry 18. A method for manufacturing a compound according to entry    1, wherein the compound is manufactured from two or more different    chemical elements, where the types and quantities of the chemical    elements are chosen such that    -   the resultant compound crystallizes into cubic crystals whose        lattice constants differ by no more than 10%,    -   the compound belongs to the class of Heusler phases, and    -   the compound has an electronic instability in the vicinity of        (within ±0.5 eV of) the Fermi energy.-   Entry 19. A method according to entry 18, wherein the types and    quantities of the chemical elements are chosen such that the    resultant compound has a valence electron concentration of 6.95±0.5.-   Entry 20. A method according to entry 18, wherein the types and    quantities of the chemical elements are chosen such that the    resultant compound has a valence electron concentration of 5.5±0.5.-   Entry 21. A method according to entry 18, wherein the types and    quantities of the chemical elements are chosen such that the    resultant compound has a valence electron concentration of 7.13±0.5.-   Entry 22. A method according to entries 18-21, wherein the compound    is doped with electrons or holes.-   Entry 23. A method according to any of entries 18-22, wherein the    types and quantities of the chemical elements are chosen such that    the resultant compound has    -   (a) a local magnetic moment and    -   (b) a Curie temperature in excess of room temperature.-   Entry 24. A method according to any of entries 18-23, wherein the    types and quantities of the chemical elements are chosen such that    the resultant compound is a ferromagnetic semimetal.-   Entry 25. Employment of compounds or composites according to any of    entries 1-17 for manufacturing read-heads employed on storage    devices.-   Entry 26. Employment of compounds or composites according to any of    entries 1-17 as magnetic-field sensors.-   Entry 27. Employment of compounds or composites according to any of    entries 1-17 in spin electronics.-   Entry 28. Employment of compounds or composites according to any of    entries 1-17 for manufacturing TMR-devices.-   Entry 29. Employment of compounds having cubic symmetry, a combined    PMR-effect, and an intrinsic field responsivity >10% per 0.1 T as    magnetic-field sensors.-   Entry 30. Employment of compounds having cubic symmetry, combined    GMR/CMR-PMR-effects, and an intrinsic field responsivity >10% per    0.1 T in spin electronics.-   Entry 31. Employment of compounds according to entry 30 for    manufacturing TMR-devices.

1. A method for manufacturing an inorganic intermetallic compound, themethod comprising the following operations: pressing measured quantitiesof a least two different chemical elements into pellets; fusing thepellets for about 30 seconds in an inert-gas atmosphere using an arcwelder; and choosing the types and quantities of the chemical elementsso that: the compound exhibits a PMR-effect; the compound has anintrinsic field responsivity greater than 10% per 0.1 T at temperaturesin excess of 290 K; the resultant compound crystallizes into cubiccrystals whose lattice constants differ by no more than 10%; thecompound belongs to the class of Heusler phases; and the compound has anelectronic instability within ±0.5 eV of the Fermi energy.
 2. The methodaccording to claim 1, wherein the operations further comprise doping thecompound with electrons or holes.
 3. The method according to claim 1,wherein the types and quantities of the chemical elements are alsochosen such that the resultant compound has a valence electronconcentration of 6.95±0.5.
 4. The method according to claim 1, whereinthe types and quantities of the chemical elements are also chosen suchthat the resultant compound has a valence electron concentration of5.5±0.5.
 5. The method according to claim 2, wherein the types andquantities of the chemical elements are also chosen such that theresultant compound has a valence electron concentration of 7.13±0.5. 6.The method according to claim 2, wherein the types and quantities of thechemical elements are also chosen such that the resultant compound is aferromagnetic semimetal, that has a local magnetic moment, and a Curietemperature in excess of room temperature.